(640g) Effect of PMMA Particle Size on Coating Behavior in Combined Gliding Arc Discharge and Spouted Bed with Solid Precursor

A novel particle coating method using zinc particle as a solid precursor and gliding arc discharge was used to coat poly methyl methacrylate (PMMA) particles in spouted bed. An AC voltage was applied to two diverging electrodes to generate the arc discharge. A flow rate of 1 L·min^-1 of argon gas was used to elongate the arc discharge as well as to establish a spouted bed condition inside the reactor. By using a combination of gliding arc discharge and spouted bed condition, a particle coating performance was achieved. Coating process was performed in a small scale reactor with a dimension of 75 mm × 30 mm × 3 mm. Both zinc solid precursor and PMMA particle were pre-loaded inside the reactor before the coating process. The coated samples were collected after a certain time intervals and were analyzed by scanning electron microscope (SEM) to observe the coating morphology and thickness. The energy dispersive X-ray spectroscopy (EDS) was also used to calculate the coverage of the coating layer, while an inductively coupled plasma optical emission spectrometry (ICP-OES) was used to measure the amount of zinc coating and indirectly used to calculate the thickness of the coating layer.

In previous study, two coating mechanisms were proposed for this novel particle coating process, which are the coating by physical contact with the partially melted zinc particle and coating by vapor deposition. For this research, we will focus on confirming the proposed coating mechanisms by studying the effect of PMMA particle size on the coating behavior. Two different sizes of PMMA particles were used, 0.3 and 0.6 mm in diameter. The increase in the PMMA particle size decreased the terminal settling velocity of zinc over PMMA (TSV) ratio from 1.91 to 1.35. The frequency of collision between zinc and PMMA particle will be affected by the change in the TSV ratio. Two applied voltages, 1.5 and 4.5 kV, was also studied to observe the changed in the coating mechanisms.

A fine layer of zinc coating was observed by SEM after 5 min. The results showed that all conditions tested exhibited similar trend for the amount of zinc coating as a function of coating time. The amount of zinc coating increased in the early stage of the coating process and became relatively steady afterward due to the peeling off of the coating layer. The amount of zinc coating dropped by approximately 50% when the diameter of the PMMA was doubled, regardless of the applied voltage. This is due to the decrease in the surface area to volume ratio of the PMMA particle.

The coating behavior depended significantly on the applied voltage. At 1.5 kV, the amount of zinc coating was higher than that of the coated sample obtained at 4.5 kV. Moreover, the coating seemed to be thicker when 1.5 kV of applied voltage was used to generate the gliding arc discharge. This is because, the energy provided can partially melt and partially vaporized the solid precursor allowing a mixed of coating mechanisms, coating by physical contact with the partially melted zinc particle and coating by vapor deposition. Due to the significant amount of coating obtained by physical contact with the partially melted zinc particle, it resulted in thick and rough coating layer. In addition, at low applied voltage, the amount of zinc coating was observed to increase when increasing the TSV ratio as the frequency of collision between zinc and PMMA was increased when TSV ratio was increased. This result in an increase in the rate of coating by physical contact with the partially melted zinc particle. On the other hand, at 4.5 kV, the energy provided by the gliding arc discharge was high enough to totally vaporize the solid precursor allowing mainly coating by vapor deposition. Hence, at this applied voltage, the amount of coating as well as coating thickness was independent of the TSV ratio.

The coated samples had an average thickness in the range of 100 to 200 nm. In the case of zinc percentage coverage, it is indicated that both applied voltage and TSV ratio did not have significant effect on the zinc percentage coverage. This means that the major contributor for the peeling off of the coating layer was the collision between PMMA particles and attack from gliding arc discharge rather than the collision between zinc and PMMA particles.